It has been proposed to make regular use in wavelength division multiplex fiber optic transmission systems of periodic regeneration of signals by synchronous modulation. The modulation is preferably optical, especially in high bit rate systems. The problem then arises of group velocity differences between the channels due to the differences between the wavelengths and the dispersion of the line optical fibers. The group velocity differences desynchronize the bit times of the various channels and prevent simultaneous synchronous modulation of the channels of the multiplex.
A first solution to the problem is to separate the various channels before using synchronous modulation to regenerate each channel. The regenerated channels can then be multiplexed. A regenerator in that configuration is made up of the same number of synchronous modulators as there are channels, connected in parallel. Because the channels are not synchronized each regenerator has its own clock recovery circuit. That solution is bulky, costly and has a high power consumption because of the number of electronic circuits to be replicated for clock recovery.
French Patent Application 99 02 126 filed on Feb. 2, 1999 (Fo. 102078) proposes to regenerate only a subset of channels in a regenerator, for example one channel in four. All channels are processed downstream of a plurality of regenerators, four regenerators in that example. That solution reduces the number of modulators to be connected in parallel in each regenerator, by a factor of four in that example. For the same performance, the total number of modulators in the transmission system remains the same. That solution simplifies the structure of a regenerator but has no effect on cost or power consumption.
Other solutions to the problem of synchronous regeneration entail synchronizing the various channels periodically in order to regenerate the channels simultaneously at the point of synchronicity. An article by E. Desurvire, O. Leclerc and O. Audouin entitled “Synchronous in-line regeneration of wavelength division multiplexed solitons signals in optical fibers”, Optics Letters, vol. 21, no. 14, pages 1026-1028, describes a scheme for allocating wavelengths that is compatible with the use of synchronous modulators for soliton signals. The article proposes allocating wavelengths to the various channels of the multiplex so that, for given intervals ZR between repeaters, the signals of the various channels, or to be more precise the bit times of the various channels, of the multiplex are substantially synchronized on reaching the repeaters. That enables in-line synchronous modulation of all channels at given intervals using discrete synchronous modulators. That technique of allocating the wavelengths of the multiplex is also described in French Patent Application FR-A-2 743 964 in the name of Alcatel Submarine Networks. The article proposes choosing a sub-group of channels that are synchronous not only with intervals ZR but also with intervals which are sub-multiples of ZR.
An article by O. Leclerc, E. Desurvire and O. Audouin entitled “Synchronous WDM soliton regeneration: towards 80-160 Gbit/s transoceanic systems”, Optical Fiber Technology, 3, pages 97-116 (1997), specifies that the above wavelength allocation scheme can lead to excessively large intervals ZR between the synchronous modulators or to excessively large spacings between the channels of the multiplex. To alleviate that problem, the article notes that in that kind of wavelength allocation scheme the bit times of the subsets of channels of the multiplex are synchronous with intervals that are sub-multiples of ZR. The article consequently proposes regenerating subsets of channels of the multiplex at smaller intervals. However, that solution imposes filtering of the channels of the subset to be regenerated and the transmission system loses the benefit of a single period for all channels.
FR-A-2 770 001 proposes using a synchronous modulator to modulate the soliton signals of all the channels of a wavelength division multiplex transmission system at a frequency N/T which is a multiple of the clock frequency 1/T of the signals. That loosens the synchronicity constraint by requiring the various channels to be synchronized to a sub-multiple of the bit time, instead of imposing synchronization of the bit times.
FR-A-2 759 516 proposes demultiplexing the various channels and applying the necessary time-delays to them to resynchronize them. After multiplexing, the various channels can be modulated by a single optical modulator. French Patent Application 97 06 590 proposes using a chain of gratings formed in a fiber to apply suitable time-delays and thereby resynchronize the various channels without having to employ demultiplexing and remultiplexing. An article by M. Nakazawa et al. entitled “160 Gbit/s WDM (20 Gbit/s×8 channels) soliton transmission over 10 000 km using in-line synchronous modulation and optical filtering”, Electronics Letters, vol. 34, no. 1 (1998), pp. 103-104, proposes resynchronizing the channels of a wavelength division multiplex using a different dispersion compensating fiber (DCF) for each channel. The article proposes using four modulators to regenerate eight channels, three of the modulators processing two channels simultaneously.
The above solutions have the following drawbacks. The passive solutions (choice of wavelengths or application of fixed time-delays) have the drawback of high sensitivity to the characteristics of the transmission system and in particular to dispersion, temperature, and regenerator positioning. There is additionally the problem of aging of the systems and that of correlated variations in the above characteristics.